Various kinds of offset plate precursors for direct photoengraving have been proposed and put to practical use these days. Typically, a technique of producing offset printing plates has been used widely in the field of light printing, in which a photoreceptor having, on the conductive support, a photoconductive layer consisting essentially of photoconductive grains of, for example, zinc oxide and a binder resin is first processed by ordinary electrophotography to form an highlyoleophilic toner image on its surface and thereafter the thus-processed surface is treated with an etchant to selectively render the non-image area of the surface hydrophilic.
To obtain good prints, it is necessary that an original is faithfully duplicated on an offset plate precursor, that the surface of the photoreceptor of the plate precursor is well compatible with an etchant, and that the non-image area in the image-duplicated surface is rendered sufficiently hydrophilic and has high water resistance. In addition, in actual printing with the printing plate, it is further necessary that the surface conductive layer having an image thereon is not peeled off, that the layer is well compatible with a wetting water, and that the hydrophilicity of the non-image area is well maintained so as not to cause background staining even when an increased number of prints are obtained by the printing.
It has already been known that the proportion of the zinc oxide and the binder resin in the photoconductive layer has an influence on the above-mentioned requirements. For instance, if the ratio of the binder resin to the zinc oxide grains in the photoconductive layer is reduced, the etchability of the surface of the photoconductive layer will be improved to cause no background staining, while, however, the internal cohesive force of the photoconductive layer itself will be lowered to worsen the printing durability due to the insufficiency of the mechanical strength of the layer. On the contrary, if the ratio of the binder resin to the same is increased, the printing durability will be improved, while, however, the background staining will be increased. Though, the background staining is a phenomenon that especially depends upon the etchability of the surface of the photoconductive layer, it has been clarified that the etchability is not influenced only by the proportion of the zinc oxide and the binder resin in the photoconductive layer but is also influenced noticeably by the kind of the binder resin used.
Especially in offset printing plates, the background staining to be caused by the insufficient etchability of the surface of the photoconductive layer is a serious problem, as mentioned above. In order to overcome the problem, therefore, various binder resins for zinc oxide that may improve the etchability of the surface of the photoconductive layer have been investigated and developed. For instance, JP-B 50-31011 (the term "JP-B" as referred to herein means an "examined Japanese patent publication") mentions that a combination of a copolymer resin obtained by copolymerization of (meth)acrylate monomers and other monomers in the presence of fumaric acid and having a weight average molecular weight of from 1.8.times.10.sup.4 to 10.times.10.sup.4 and a glass transition point (Tg) of from 10.degree. C. to 80.degree. C. and a copolymer comprising (meth)acrylate monomers and monomers other than fumaric acid is effective in improving the etchability of photoconductive layers; JP-A 53-54027 (the term "JP-A" as referred to herein means an "unexamined published Japanese patent application") mentions that a ternary copolymer containing substituted (meth)acrylates having a carboxylic group separated from the ester bond by at least seven carbon atoms is effective in the same; JP-A 54-20735 and 57-202544 mention that a tetranary or pentanary copolymer containing acrylic acids and hydroxyethyl (meth)acrylates is effective in the same; and JP-A 58-68046 mentions that a ternary copolymer containing (meth)acrylates substituted by an alkyl group having from 6 to 12 carbon atoms and vinyl monomers containing a carboxylic acid residue is effective in the same.
However, even the above-mentioned resins that have been said to be effective in improving the etchability of photoconductive layers were still unsatisfactory with respect to the background staining resistance and the printing durability, when they were tested and actually used.
Recently, a technique of employing, as a binder resin for a photoconductive zinc oxide, a combination of a low polymer having a particular polar group such as a carboxyl, sulfo or phosphono group as the substituent in the main chain of the polymer or at one end of the same and having a weight average molecular weight of not more than 2.times.10.sup.4 (for example, refer to JP-A 63-217354, and U.S. Pat. Nos. 4,968,572, 4,954,407, 5,134,051, 5,154,997) and various middle or high copolymers having a weight average molecular weight of not less than 3.times.10.sup.4 has been disclosed to improve the etchability of the photoconductive layer with increasing the mechanical strength of the whole photoconductive layer and improving the printing durability of the same, over the photoconductive layer containing a conventional binder resin.
As the middle or high copolymers, for example, known are random copolymers containing at least methacrylates and carboxyl group-containing monomers (for example, refer to JP-A 63-220148, and U.S. Pat. No. 4,968,572), random copolymers containing thermosetting/photosetting group-containing comonomers, which are hardened in the presence of a crosslinking compound after the photoconductive layer has been formed (for example, refer to JP-A 1-100554, 1-211766, 1-102573), etc.
The disclosed electrophotolithographic printing plate precursors were excellent, when actually used, in the reproducibility of the duplicated image. In addition, when the precursors were etched to give offset printing plates and were used in actual printing, the printing plates were excellent as having improved background staining resistance and printing durability.
On the other hand, recently, a so-called full-automatic printing machine equipped with an etching device has been popularized for the purpose of improving the printing operation efficiency. With the popularization, the smoothness of the back surface of the support of the electrophotolithographic plate precursor to be fitted to the device has become restrained so as to prevent the precursor from slipping, in order that the photoengraved plate (having thereon a duplicated image formed by electrophotoengraving) may be suitably located at a determined position in the full-automatic printing machine.
It has been found, however, even a photoreceptor having a photoconductive layer on the support having such a particular back surface often causes background staining if its photosensitive surface is strongly rubbed against the back surface of the support.
The problem was especially remarkable when a photoreceptor was fed to an electrophotoengraving machine as a sheet photoreceptor.
As well known, the surface of the lithographic plate precursor using such a photoconductive zinc oxide is rendered hydrophilic by chemically treating the zinc oxide with an etchant under an acidic condition. As an etchant of high ability, one containing yellow prussiate of potash (potassium ferrocyanide) as the main component has been essentially employed in practical use.
Since it has become necessary to restrict the method of treating the waste liquid derived from the etchant containing yellow prussiate of potash as the main component, it is desired to use a different etchant containing phytic acid as the main component. However, as being inferior to the former etchant containing yellow prussiate of potash with respect to the etchability, the latter etchant often causes black pepper stains in the non-image area and is therefore limitative with respect to the suitable etching conditions therewith. Under the situation, the improvement of the capacity of an etchant consisting essentially of phytic acid is desired and, in addition, the development of a lithographic plate precursor using a photoconductive zinc oxide, which may be well etched with such an etchant consisting essentially phytic acid, is also desired.